During manufacture of the components making up a rotor module, such as a fan, compressor or turbine assembly in a gas turbine, efforts are made to minimise mass imbalances in the individual components. Nevertheless, mass imbalances will tend to arise in the completed module, for example due to manufacturing tolerances on rotor blades. Consequently, the module as a whole must undergo a balancing operation in order to prevent subsequent stress and vibration during operation of the module.
The vibration that is of principle concern is generally the resonant vibration. This has a maximum amplitude when the speed of rotation of the rotor is at a critical speed (i.e. rotational speed matching natural frequency). Imbalances within the spool tend to drive the resonant vibration and increase its amplitude. This may cause excessive wear and stress on bearings for the spool, as well as its rotors. The balancing operation thus reduces imbalances and/or balances to compensate for them.
A spool of a gas turbine engine may be made up of plural modules, each treated independently from a balancing point of view. For example, one module can be a high pressure compressor assembly and another can be a high pressure turbine assembly, but a “module” can also be a part thereof, i.e. a sub-assembly. Each module on the spool is typically balanced in its own right, rather than balance being simply obtained across the whole spool. This allows individual modules to be substituted without a need to re-balance the rest of the spool.
In balancing a module the following sources of imbalance are generally considered and compensated for:                1) Unbalance that arises within the module due to geometric errors, such as blades being slightly different weights, or the rotor being slightly radially mal-positioned with respect to the axis of rotation.        2) Unbalance that arises within the module due to the module's geometric error at its interface with another module. In particular, the interface may not be square as a consequence of the sum of miss-alignments of sub-units of the module.        3) Unbalance that arises outside of the module (that is, in the adjacent module) due to the module's geometric error at its interface with that other module.        
To balance each module in its own right, an unbalance caused by the module must be corrected on a balancing plane within that module, even if the unbalance is of type 3).
A two plane balancing correction is typically carried out by addition or removal of mass from the module at two axially spaced-apart balancing planes which extend perpendicular to the geometric axis of the assembly. In particular, weight be added or removed from axially spaced balancing lands, which are usually located at respective ends of the module. This is achieved using a balancing machine, on which the module is rotated and its imbalances are measured. To account for unbalance of type 3), a mass simulator to simulate the (balanced) adjacent module may be used on the balancing machine.
Additionally or alternatively, imbalances can be reduced with particular build techniques such as: component balancing (balancing each component of the module), straight build (eliminating as far as possible the geometric errors that give rise to type 2) and 3) unbalance through careful building up of the sub-units of the module), and blade distribution (arranging the blades of different weights to better balance one another).
A difficulty with the use of balancing lands is that they may be significantly axially spaced from the unbalance that they are compensating for, especially in the case of type 3) unbalance. This axial spacing between the unbalance and its compensating balancing mass creates a bending moment, which may not be detectable at the low rotation speed of the balancing machine. If the bending moment results in flexing at higher rotation speeds, this can create a new imbalance that may drive the resonant vibration.
It would be desirable to perform balancing operations in a more efficient manner, which avoids, as far as possible, unnecessary expensive balancing activities (e.g. component balancing, straight build and/or blade distribution work) and also avoids the production of bending moment which cause flexing at higher rotation speeds.